Synthesis and characterization of thermally stable second-order nonlinear optical side-chain polyurethanes containing nitro-substituted oxadiazole and thiazole chromophores

Article abstract of DOI:10.1016/j.molstruc.2011.04.032

We have newly synthesized nonlinear optical (NLO) active nitro-substituted thiazole and oxadiazole chromophores and condensed with tolylene-2,4- diisocyanate and 4,4′-methylenedi(phenyl isocyanate) to yield a series of polyurethanes. Molecular structural characterization of the resulting chromophores and polyurethanes was achieved by FTIR, UV-vis, ¹H NMR and CHN elemental analyzer. The inherent viscosities (η inh) of polyurethanes measured with an Ubbelohde viscometer were in the range of 0.26-0.30 dl/g. Thermal behavior of polyurethanes was investigated using differential scanning calorimetry and thermogravimetric analysis. The glass transition temperatures (T_g) of the polyurethanes were in the range of 121-192 °C. Thin films of polyurethanes were prepared and achieved molecular orientation by inducing electric field. The change in the surface morphology of polyurethanes films before and after poling was investigated using atomic force microscopy. All the polyurethanes exhibited an excellent solubility in most of the common organic solvents, suggesting that these polyurethanes offered good processability. The second harmonic generation (SHG) coefficients (d₃₃) of the poled polyurethanes ranged from 29.7 to 44.2 pm/V at 532 nm. High thermal endurance of the poled dipoles was observed for all the polyurethanes and this was attributed to the formation of extensive hydrogen bonds between urethane linkages. Furthermore, none of the developed polyurethanes showed SHG decay below 115 °C, and this signified their acceptability for nonlinear optical devices.

Full text of DOI:10.1016/j.molstruc.2011.04.032

Journal of Molecular Structure 1000 (2011) 10–23
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of thermally stable second-order nonlinear
optical side-chain polyurethanes containing nitro-substituted oxadiazole
and thiazole chromophores
⇑
R.G. Tasaganva, S.M. Tambe, M.Y. Kariduraganavar
Center of Excellence in Polymer Science and Department of Chemistry, Karnatak University, Dharwad 580 003, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We have newly synthesized nonlinear optical (NLO) active nitro-substituted thiazole and oxadiazole
chromophores and condensed with tolylene-2,4-diisocyanate and 4,40-methylenedi(phenyl isocyanate)
to yield a series of polyurethanes. Molecular structural characterization of the resulting chromophores
and polyurethanes was achieved by FTIR, UV–vis, 1H NMR and CHN elemental analyzer. The inherent vis-
Received 4 October 2010
Received in revised form 15 April 2011
Accepted 18 April 2011
Available online 27 April 2011
cosities (g inh) of polyurethanes measured with an Ubbelohde viscometer were in the range of 0.26–
0.30 dl/g. Thermal behavior of polyurethanes was investigated using differential scanning calorimetry
and thermogravimetric analysis. The glass transition temperatures (Tg) of the polyurethanes were in
the range of 121–192 °C. Thin films of polyurethanes were prepared and achieved molecular orientation
by inducing electric field. The change in the surface morphology of polyurethanes films before and after
poling was investigated using atomic force microscopy. All the polyurethanes exhibited an excellent sol-
ubility in most of the common organic solvents, suggesting that these polyurethanes offered good pro-
cessability. The second harmonic generation (SHG) coefficients (d33) of the poled polyurethanes ranged
from 29.7 to 44.2 pm/V at 532 nm. High thermal endurance of the poled dipoles was observed for all
the polyurethanes and this was attributed to the formation of extensive hydrogen bonds between ure-
thane linkages. Furthermore, none of the developed polyurethanes showed SHG decay below 115 °C,
and this signified their acceptability for nonlinear optical devices.
Keywords:
NLO chromophore
Polyurethane
Corona poling
Thermal stability
SHG coefficient
Order parameter
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
design of selective molecules with large second-order nonlinear
optical (NLO) responses has been the subject of today’s research.
NLO polymers possess vast potential for use in a variety of pho-
tonics systems, including high speed optical modulators, ultrafast
optical switches, and high density optical data storage media [1–
5]. These devices are essential for continued advancement in the
effort to transform information storage and transmission from
the electrical to the optical regime. The promise of NLO polymers
lies in their fortuitous combination of exceptional optical qualities,
low cost and ease of fabrication into device structures [6–10]. This
juxtaposition of technologically favorable characteristics has led to
a consideration research into the development of NLO polymers for
commercial application.
Although second-order NLO polymers hold promising applica-
tions in optic devices, a number of issues have to be addressed
thoroughly before they possess commercial value. Three of these
crucial issues are the high temporal stability of dipole orientation,
large optical nonlinearity and minimum optical loss. Therefore, the
Generally, when new NLO materials are being designed, the
intrinsic NLO efficiency of the chromophore segments is mainly
considered as a relevant parameter [11]. Recently, it has been
proved that the introduction of aromatic heterocyclic rings along
the conjugation path results in increased hyperpolarizability in
comparison with all benzenoid systems [12–14]. This is due to
delocalization energy of heteroaromatics, which is lower than
that of six membered rings. Chromophores containing penta aro-
matic heterocycles or their derivatives, such as thiophene [15–
17], thiazole [18,19], benzothiazole [20] and benzoxazole [21],
are among the most studied systems. In addition, it is also nec-
essary to give primary importance to other ancillary properties
that may play an important role by influencing material process-
ing and performance (e.g., film making, poling deficiency,
durability, and thermal and photochemical stability) [22]. To
achieve these, several strategies have been proposed recently.
Extensively crosslinked polymeric matrices [23–26] and high
glass transition materials such as polyimides [27–31] have been
studied to suppress the orientational relaxation at high operation
temperature. The most recent strategy for satisfying high
⇑
Corresponding author. Tel.: +91 836 2215286; fax: +91 836 2771275.
E-mail address: mahadevappak@yahoo.com (M.Y. Kariduraganavar).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.04.032

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
11
Scheme 1. Synthesis of chromophores.
orientational stability is the development of NLO polymers,
whose dipole moments align transversely to the polymer back-
bone [32–34].
In this series of NLO polymers, polyurethanes with NLO chro-
mophores found to be promising candidates, whose dipole mo-
ments are aligned transverse to the main-chain backbones,

12
R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
exhibiting large second-order nonlinearity with good thermal sta-
bility [32,33,35]. Furthermore, systems of pseudo-crosslinked via
hydrogen bonds have the advantages such as homogeneity and
good processability in comparison with chemically cross-linked
Scheme 2. Synthesis of polyurethanes.

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
13
Scheme 2 (continued)
systems, which suffer from significant optical loss and poor pro-
2.2. Instrumentation
cessability. It has also been reported [32–36] that crosslinked poly-
urethanes with azo-benzene chromophore show enhanced
thermal stability as the polymer matrix forms extensive hydrogen
bonds between urethanes linkages and thereby increases the rigid-
ity, preventing the relaxation of induced dipoles [19,37–40].
Realizing the importance as discussed above, we have made an
attempt to synthesize nitro-substituted thiazole and oxadiazole
chromophores and condensed with tolylene-2,4-diisocyanate and
4,40-methylenedi(phenyl isocyanate) to achieve better NLO perfor-
mance and processability. The oxadiazole heteroaromatic ring was
particularly chosen on account of its well known features of ther-
mo-chemical stability [13,41]. The physico-chemical properties of
the resulting chromophores and the polymers were studied using
different techniques. Further, we discuss the details of their optical
second-order NLO activity in terms angular dependence, temporal
stability along with thermal stability.
Fourier transform infrared (FTIR) spectra were recorded by a
Nicolet-5700 spectrometer. The proton nuclear magnetic reso-
nance (1H NMR) spectra were recorded on a Bruker Avance
300 MHz spectrometer using tetramethylsilane (TMS) as reference.
Ultraviolet–visible (UV–vis) spectra were measured using Hitachi
U-2800 spectrophotometer with an accuracy of 0.3 nm. Differen-
tial scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) were performed respectively on a Mettler-Toledo DSC
822e and Perkin–Elmer Diamond TGA/DTA thermogravimetric
analyzer at a heating rate of 10 °C/min in a nitrogen atmosphere.
Elemental analyses were performed with a Perkin–Elmer PE 2400
CHN elemental analyzer. The surface morphology of thin films be-
fore and after poling was investigated with Veeco diInnova SPM
atomic force microscope (AFM). The melting points were recorded
with open capillary tubes on a Mel-Temp apparatus. The inherent
viscosity of polyurethanes was measured at 25 °C using Ubbelohde
viscometer in DMF (concentration was 0.5 g/dl). Thin polymer
films were produced onto an indium–tin oxide (ITO) glass sub-
strate using a spin casting technique at a rate of about 1500–
2000 rpm from 5 wt.% solution of the polymer in DMF. Prior to film
2. Experimental section
2.1. Materials
Tolylene-2,4-diisocyanate (TDI), 4,40-methylenedi(phenyl iso-
cyanate) (MDI) and N-phenyldiethanolamine were purchased from
Sigma–Aldrich and used as received. Hydrazine hydrate, N,N-
dimethylformamide (DMF) and 4-nitrobenzoic acid were procured
from Loba Chemie, Mumbai, India. The carbon tetrachloride was
purified by distillation over phosphorous pentoxide. Cynogen bro-
mide, 4-nitroacetophenone, thioacetamide, thiourea, triphenyl
phosphine and N-bromosuccinimide were purchased from S.D.
Fine Chem. Ltd., Mumbai, India. All other solvents and reagents
were obtained commercially and used as received unless otherwise
stated.
casting, the polymer solution was filtered through a 0.20 lm Teflon
membrane filter. All films were dried for 3 h in a vacuum oven at
80 °C to remove the residual solvents. The thickness and refractive
indices of the polyurethanes were measured using a SE 850 Ellips-
ometer with an accuracy of 0.001
respectively.
lm and 0.0005 units,
The dried film was then heated up to a glass transition temper-
ature and corona poled with an intense dc electric field as de-
scribed in our earlier papers [19,42,43]. After being poled for 1 h,
the polymer film was cooled down to measuring temperature (25
and 100 °C) in the presence of electric field. The poling conditions

14
R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
Fig. 1. 1H NMR (DMSO-d6) spectrum of chromophore a.
The second harmonic measurements were performed utilizing
the setup described in our previous papers [19,42,43]. A Mode-
Locked Nd:YAG laser (Continuum Minilite-I, 6 ns pulse duration,
28 mJ maximum energy at 1064 nm, 10 Hz repetition rate) was
used as a fundamental light source. The second harmonic signal
generated by the p-polarized fundamental wavelength (1064 nm)
was detected by fast photodiode (FDS010, rise time 0.9 ns, Thorl-
abs) and an oscilloscope (Tektronix TDS 724D, Digital Phosphor
Oscilloscope) with a frequency of 500 MHz. The polymer sample
was held at 45° angle to the incident laser beam to get maximum
SHG output. A standard potassium dideuterium phosphate (KDP)
crystal was used as a reference sample.
2.3. Synthesis of chromophores
2.3.1. Synthesis of 2-bromo-1-(4-nitrophenyl) ethanone (1)
4-Nitroacetophenone (16.5 g, 0.1 mol) was dissolved in 50 ml of
chloroform and to this, 200 mg of anhydrous aluminum trichloride
(AlCl3) was added at once. It was then kept for stirring at around
45 °C. A bromine solution (0.15 mol), which was previously diluted
in chloroform 20 ml, was added drop-wise for 1 h. The completion
of reaction was confirmed by TLC. The solvent was removed under
reduced pressure. The residual solid was filtered and crystallized
from methanol and afforded 20.0 g (81.96% yields) of 1. M.p.: 90–
92 °C. FT-IR (KBr, cmꢀ1): 3111, 3021 (Ar–H C–H) 2885 (Aliph.C–
H), 1698 (C@O), 1530, 1321 (N@O). 1H NMR (300 MHz, CDCl3):
d = 6.6 (s, 2H), 8.3 (q, 4H). Anal. Calcd. for C8H6BrNO3: C, 39.37;
H, 2.48; N, 5.74. Found: C, 39.61; H, 2.35; N, 5.56%.
2.3.2. 2-Methyl-4-(4-nitrophenyl) thiazole (2)
Fig. 2. FTIR spectra of polyurethanes Ic, IIc, Ib and IIb.
A mixture of 2-bromo-1-(4-nitrophenyl) ethanone 1 (4.88 g,
20.0 mmol) and thioacetamide (1.5 g, 20.0 mmol) was dissolved
in 100 ml of ethanol and allowed to reflux for 5 h. While refluxing
were as follows: high voltage, 4 kV at the needle point; gap dis-
tance, ca. 0.8 cm; poling current, <0.25 mA.

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
15
Fig. 3. 1H NMR (DMSO-d6) spectrum of polyurethane IIa.
(KBr, cmꢀ1): 3050, 2924 (Ar–H C–H) 2855 (Aliph.C–H), 1604
(C@N), 1517, 1342 (N@O). 1H NMR (300 MHz, CDCl3): d = 4.8 (s,
2H), 7.5 (s, 1H), 8.2–8.3 (q, 4H). Anal. Calcd. for C10H7BrN2O2S: C,
40.15; H, 2.36; N, 9.36. Found: C, 40.76%; H, 2.39%; N, 9.29%.
Table 1
Physical and NLO properties of polyurethanes.
inha kmax
Topt’s Uc
Film thickness d33 (pm/V)
(lm)
b
Polyurethanes
g
(nm) (°C)
Ia
IIa
Ib
IIb
Ic
IIc
0.28
0.30
0.27
0.29
0.26
0.28
436
455
463
432
452
410
115
134
170
160
185
140
0.14 0.22
0.34 0.24
0.21 0.31
0.18 0.22
0.17 0.28
0.24 0.30
29.7
31.5
30.2
40.9
41.6
44.2
2.3.4. Synthesis of phosphonium salt (4)
A
mixture of 2-(bromoethyl)-4-(4-nitrophenyl) thiazole 3
(3.2 g, 10.66 mmol) and triphenyl phosphine (2.79 g, 10.66 mmol)
was dissolved in 30 ml of dry DMF and refluxed for 20 h. After con-
firming the completion of reaction by TLC, the reaction mixture
was poured into water. The resulting brown color solid was filtered
and washed several times with benzene followed by petroleum
ether. It was dried in vacuum desiccator and afforded 3.1 g
(51.75% yields) of 4. M.p.: 110–112 °C. FT-IR (KBr, cmꢀ1): 3280,
3100 (Ar–H C–H) 2928 (Aliph.C–H), 1604 (C@N), 1522, 1344
(N@O). 1H NMR (300 MHz, CDCl3): d = 7.4–7.7 (m, 18H), 8.0 (d,
2H), 8.2 (d, 2H). Anal. Calcd. for C28H22BrN2O2PS: C, 59.90; H,
3.95; N, 4.99. Found: C, 60.20%; H, 4.01%; N, 5.02%.
a
b
c
Inherent viscosity of polymer: concentration of 0.5 g/dl in DMSO at 25 °C.
Polyurethane film after corona poling.
Order parameter.
the solid was separated out, indicating the completion of reaction.
The resulting solid was filtered and washed with hot ethanol. It
was then dried in oven at 70 °C for 48 h and afforded 3.6 g
(81.81% yields) of 2. M.p.: 132–35 °C. FT-IR (KBr, cmꢀ1): 3190
(Ar–H C–H) 2925 (Aliph.C–H), 1597 (C@N), 1502, 1307 (N@O). 1H
NMR (300 MHz, CDCl3): d = 2.8 (s, 3H), 7.5 (s, 1H), 8.0 (d, 2H), 8.2
(d, 2H). Anal. Calcd. for C10H8N2O2S: C, 54.54; H, 3.66; N, 12.72.
Found: C, 54.81; H, 3.58; N, 12.69%.
2.3.5. Synthesis of chromophore (a)
A solution of t-butoxide (0.639 g, 5.5 mmol) prepared in 10 ml
absolute ethanol was added into a solution of 4-[N,N-bis(2-
hydroxyethyl)amino]-benzaldehyde (1.04 g, 5 mmol), which was
previously dissolved in 10 ml of absolute alcohol. To this, an alco-
holic solution of phosphonium salt (4, 2.8 g, 5 mmol) was added
and stirred for 40 h at room temperature under nitrogen atmo-
sphere. The resulting reaction mixture was diluted with water
and the precipitate thus obtained was filtered and washed with
water. It was then crystallized from dioxane and afforded 1.2 g
(48.80% yield) of chromophores a. M.p.: 150–153 °C, Dec-
omposition Temp.:168 °C. Maximum absorption wavelength
(kmax; DMF): 409 nm. FT-IR (KBr, cmꢀ1): 3410–3350 (O–H), 3080,
2948 (Ar–H C–H) 2855 (Aliph.C–H), 1591 (C@C), 1610 (C@N),
1523, 1340 (N@O). 1H NMR (300 MHz, DMSO-d6): d = 3.5 (t, 4H),
3.8 (t, 4H), 4.2 (s, 2H), 7.4 (d, 1H), 7.5 (d, 1H), 7.6–7.7 (m, 5H),
2.3.3. 2-(bromoethyl)-4-(4-nitrophenyl) thiazole (3)
A mixture of 2-methyl-4-(4-nitrophenyl) thiazole 2 (4.0 g,
18.56 mmol) and N-bromosuccinimide (3.3 g, 18.56 mmol) was
dissolved in carbon tetrachloride. To this, a catalytic amount of
2,20-azobisisobutyronitrile (AIBN) was added as an initiator. The
reaction mixture was then refluxed for 8 h under nitrogen atmo-
sphere. The completion of the reaction was confirmed by the
appearance of succinimide on the top of the reaction solution. A
yellow solution was obtained after filtration of the succinimide.
The filtrate was then poured into water and extracted with meth-
ylene chloride. The organic layer thus collected was dried over
MgSO4, and finally the solvent was removed using a rotary evapo-
rator. The crude product was purified by crystallization in metha-
nol. The yield of 3 was 3.2 g (59.25% yields). M.p.: 92–94 °C. FT-IR

16
R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
C9H7N3O2S: C, 48.86; H, 3.19; N, 19.00. Found: C, 48.96%; H,
3.08%; N, 19.11%.
2.3.7. Synthesis of 5-nitro-4-(4-nitrophenyl) thiazole-2-amine (6)
While stirring, 4-(4-nitrophenyl) thiazole-2-amine (4.4 g.
20.0 mmol) was slowly dissolved in 20 ml of conc. H2SO4 so as to
maintain the temperature between 8 and 12 °C. The resulting solu-
tion was cooled to 4 °C and to this, 1 ml (20.0 mmol) of 90% nitric
acid was added while stirring at such a rate that the temperature
remains constant. The reaction mixture was then brought to room
temperature and kept for overnight. The precipitate thus collected
was filtered and washed with water till the filtrate became neutral.
The resulting product 6 was found to be 4.7 g (90.38% yields). M.p.:
251–52 °C. FT-IR (KBr, cmꢀ1): 3400 (N–H), 2859 (Aliph.C–H), 1620
(C@N), 1538,1342, (N@O). 1H NMR (300 MHz, DMSO-d6): d = 4.2 (s,
2H), 8.4 (d, 2H), 8.6 (d, 2H). Anal. Calcd. for C9H6N4O4S: C, 40.60; H,
2.27; N, 21.04. Found: C, 40.55%; H, 2.33%; N, 21.11%.
2.3.8. Synthesis of chromophore (b)
Dry sodium nitrite (1.38 g, 20.0 mmol) was slowly added into
cold conc. H2SO4 acid (1.2 ml) and allowed to warm (ꢁ50 °C) on
a water bath. This was then cooled to 0 °C and to this, a mixture
(20 ml) of glacial acetic acid and propionic acid in the ratio of
17:3 was added drop-wise with constant stirring so as to maintain
the temperature below 15 °C. The resulting solution was cooled be-
low 0 °C, and to this, 4-(4-nitrophenyl) thiazole-2-amine (5.32 g,
20 mmol) was added portion-wise with stirring. The stirring was
continued further at the same temperature for 2 h. The excess ni-
trous acid (positive test on a starch–iodide paper) was decomposed
with urea. The resulting diazonium salt solution was added drop-
wise into N-phenyldiethanolamine (3.62 g, 20.0 mmol), which
was previously dissolved in 15 ml of dilute acetic acid (10 ml acetic
acid + 5 ml of water). The pH of the reaction was maintained be-
tween 4 and 5 by simultaneously addition of 10% sodium acetate
solution. The addition was carried out for almost 30 min with vig-
orous stirring at 0 °C. The stirring was continued for another 2 h
allowing the temperature was raised to ambient. The dye was then
filtered off, washed with warm and cold water until it was free
from acid, and then dried at 60 °C in an oven for almost 48 h. Crys-
tallization from ethanol provided the pure monomer b.
Fig. 4. DSC thermograms of polyurethanes at a heating rate of 10 °C/min under a
nitrogen atmosphere.
Table 2
Thermal properties of polyurethanes.
Polyurethanes aTg (°C) bDegradation temperature (°C)
bResidue (%)
at 700 °C
10%-loss
20%-loss
40%-loss
Ia
IIa
Ib
IIb
Ic
IIc
122
140
175
170
192
145
270.27
215.39
257.67
246.02
264.20
259.93
345.87
264.60
297.21
273.84
285.46
274.65
609.69
378.52
479.46
316.01
298.10
291.86
56.10
36.21
50.50
21.16
14.94
18.63
The yield of monomer b was found to be 7.5 g (82.41%). M.p.:
125–27 °C, Decomposition Temp.: 138–140 °C. Maximum absorp-
tion wavelength (kmax; DMF): 428 nm. FT-IR (KBr, cmꢀ1): 3422
(O–H), 2855 (Aliph.C–H), 1674 (C@N), 1596 (N@N), 1543, 1341
(N@O). 1H NMR (300 MHz, DMSO-d6): d = 3.4 (s, 8H), 4.2 (s, 2H),
7.9–8.8 (m, 8H). Anal. Calcd. for C19H18N6O6S: C, 49.78; H, 3.96;
N, 18.33. Found: C, 49.64%; H, 3.86%; N, 18.44%.
a
Glass transition temperature measured with DSC under the protection of N2 at a
scan rate of 10 °C/min.
b
Degradation temperature measured with TGA under the protection of N2 at a
scan rate of 10 °C/min.
8.0 (d, 2H), 8.2 (d, 2H). Anal. Calcd. for C21H21N3O4S: C, 61.30; H,
5.14; N, 10.21. Found: C, 61.54%; H, 5.22%; N, 10.18%.
2.3.9. Synthesis of 5-(4-nitrophenyl)-1,3,4-oxadiazole-2-amine (7)
In a round-bottom flask, a mixture of p-nitrophenyl hydrazide
(4.5 g. 25 mmol) and cynogen bromide (2.75 g. 26 mmol) was ta-
ken in dry methanol, which was fitted with condenser and heated
in an oil bath at around 50–60 °C under nitrogen atmosphere for
5 h. The resulting clear solution was cooled to room temperature
and neutralized with sodium bicarbonate solution. The solid thus
separated was filtered and washed with excess amount of water.
It was then dried and crystallized from a mixture of ethanol and
dioxane.
The yield of the product (7) was 2.8 g (59.57%). M.p.: 285 °C. FT-
IR (KBr, cmꢀ1): 3276 (N–H), 1659 (C@N), 1528, 1341 (N@O). 1H
NMR (300 MHz, DMSO-d6): d = 7.5 (s, 2H), 8.0 (d, 2H), 8.3 (d, 2H).
Anal. Calcd. for C8H6N4O3: C, 46.61; H, 2.93; N, 27.18. Found: C,
46.51%; H, 2.88%; N, 27.29%.
2.3.6. Synthesis of 4-(4-nitrophenyl) thiazole-2-amine (5)
A mixture of 2-bromo-1-(4-nitrophenyl) ethanone 1 (4.88 g,
20.0 mmol) and thiourea (1.5 g, 20.0 mmol) was dissolved in
100 ml of ethanol. The reaction mixture was refluxed for 10 h. After
confirmed the completion of the reaction by TLC, the reaction mix-
ture was cooled to ambient temperature and then poured into cold
water. It was then basified using liquor ammonia. The resulting
yellow solid separated was filtered and washed with copious
amount of water. It was then dried in oven at 110 °C for 48 h and
afforded 3.3 g (75.0% yield) of 5. M.p.: 190–92 °C. FT-IR (KBr,
cmꢀ1): 3399–3363 (N–H), 3107 (Ar–H C–H) 2902 (Aliph.C–H),
1604 (C@N), 1524, 1342 (N@O). 1H NMR (300 MHz, CDCl3):
d = 7.2 (s, 2H), 7.4 (s, 1H), 8.0 (d, 2H), 8.2 (d, 2H). Anal. Calcd. for

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
17
Fig. 5. TGA thermograms of polyurethanes at a heating rate of 10 °C/min under a nitrogen atmosphere.
Fig. 6. UV–vis absorption spectra of polyurethane IIa before and after poling.
2.3.10. Synthesis of chromophore (c)
2.4. Synthesis of polyurethanes
Synthesis of chromophore c was carried out as per the proce-
dure adopted for b using 5-(4-nitrophenyl)-1,3,4-oxadiazole and
afforded 5.5 g (69.62%). M.p.: >320 °C, Decomposition Temp.:
338 °C. Maximum absorption wavelength (kmax; DMF): 412 nm.
FT-IR (KBr, cmꢀ1): 3337 (O–H), 2857 (Aliph.C–H), 1644 (C@N),
1598 (C@C), 1605 (N@N), 1543, 1338 (N@O). 1H 1H NMR
(300 MHz, DMSO-d6): d = 3.4 (s, 8H), 4.9 (s, 2H), 6.9 (d, 2H), 7.8
(d, 2H), 7.9 (d, 2H), 8.3 (d, 2H). Anal. Calcd. for C18H18N6O5: C,
54.27; H, 4.55; N, 21.10. Found: C, 54.39%; H, 4.44%; N, 21.18%.
The reaction routes of chromophores a, b and c are shown in
Scheme 1.
2.4.1. Polyurethane Ia
A 50 ml capacity of two necked round bottom flask containing
chromophore a (0.82 g, 2.0 mmol) was fitted to a condenser and
it was purged with nitrogen gas continuously for 20 min to evacu-
ate the atmospheric moisture. To this, 5 ml of dry DMF and 1
equivalent of tolylene-2,4-diisocyanate (0.34 g, 2.0 mmol) were
added and the reaction mixture was heated to 110 °C for 6 h. The
temperature of the resulting viscous solution was brought to ambi-
ent temperature and then, it was poured into cold water. The pre-
cipitate thus obtained was filtered and washed several times with

18
R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
Fig. 7. AFM images of a spin-coated film of polyurethane IIc: (a) before corona-poling; (b) after corona poling.
water. Further, it was dissolved in a minimum quantity of DMF and
re-precipitated by pouring into a large quantity of methanol to re-
move the monomer, low molecular weight dimer, trimer and so on.
The resulting polyurethane Ia was filtered and dried in vacuum
desiccator. FT-IR (KBr, cmꢀ1): 2937, 2855 (Aliph.C–H), 1646
(C@O), 1598 (C@C), 1506, 1338 (N@O). 1H NMR (300 MHz,
DMSO-d6): d = 2.0 (s, 3H), 3.4 (s, br 8H), 6.7 (d, 1H), 7.2 (d, 1H),
7.5–7.6 (m, 8H), 8.1–8.2 (d, 2H), 8.2–8.3 (d, 2H), 8.5 (s, 1H), 9.0
(s, 1H). Anal. Calcd. for C30H27N5O6S: C, 61.53; H, 4.61; N, 11.96.
Found: C, 61.85; H, 4.54; N, 12.08%.
1H), 9.4 (s, 1H). Anal. Calcd. for C28H24N8O8S: C, 53.14; H, 3.79;
N, 17.72. Found: C, 53.71%; H, 3.89%; N, 17.69%.
2.4.3. Polyurethane IIa
FT-IR (KBr, cmꢀ1): 3300 (N–H), 3000, 2961, 2853 (Ar–H C–H),
1644 (C@O), 1597 (C@N), 1508, 1338 (N@O). 1H NMR (300 MHz,
DMSO-d6): d = 2.08 (s, 2H), 3.4–3.7 (s, br 8H), 6.4 (d, 1H), 6.8 (d,
1H), 7.0–7.1 (m, 4H), 7.3 (m, 4H), 7.5–7.6 (m, 5H), 8.1–8.2 (d,
2H), 8.3 (d, 2H), 8.5 (s, 1H), 9.1 (s, 1H). Anal. Calcd. for
C36H31N5O6S: C, 65.35; H, 4.68; N, 10.59. Found: C, 65.56%; H,
We have adopted similar procedure for the syntheses of poly-
urethanes Ib, IIa, IIb, Ic and IIc with same scale and therefore, only
the spectral and analytical data are given below:
4.61%; N, 10.68%.
2.4.4. Polyurethane IIb
2.4.2. Polyurethane Ib
FT-IR (KBr, cmꢀ1): 3344 (N–H), 2929, 2861 (Aliph.C–H), 1708
(C@O), 1599 (C@C), 1653 (C@O), 1555, 1335 (N@O); 1H NMR
(300 MHz, DMSO-d6): d = 2.0 (s, 2H), 3.6 (s, 4H), 3.8 (s, 4H), 7.1–
7.3 (m, 8H), 7.7–8.0 (m, 4H), 8.3 (d, 2H), 8.5 (d, 2H), 9.4 (s, 1H),
FT-IR (KBr, cmꢀ1): 3309 (N–H), 2923, 2853 (Aliph.C–H), 1652
(C@O), 1598 (C@N), 1554, 1321 (N@O). 1H NMR (300 MHz,
DMSO-d6): d = 2.0 (s, 3H), 3.4 (s, br 8H), 7.1–8.3 (m, 11H), 8.9 (s,

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
19
Fig. 8. The SHG signals of polyurethane Ib as a function of temperature.
Fig. 9. The SHG intensity of poled polyurethane Ia as a function of incident angle.
9.6 (s, 1H). Anal. Calcd. for C34H28N8O8S: C, 57.62; H, 3.95; N, 15.81.
Found: C, 57.31%; H, 3.8%; N, 15.95%.
DMSO-d6): d = 2.0 (s, 2H), 3.6 (s, 4H), 3.7 (s, 4H), 6.9–7.3 (m, 8H),
7.8–7.9 (d, 4H), 8.3–8.5 (d, 5H), 9.6 (s, 1H). Anal. Calcd. for
C33H28N8O7: C, 61.11; H, 4.32; N, 17.28. Found: C, 61.44%; H,
4.21%; N, 17.35%.
The chemical reaction routes of all the polyurethanes are pre-
sented in Scheme 2.
2.4.5. Polyurethane Ic
FT-IR (KBr, cmꢀ1): 3333 (N–H), 2962, 2852 (Aliph.C–H), 1704
(C@O), 1600 (C@N), 1516, 1332 (N@O). 1H NMR (300 MHz,
DMSO-d6): d = 2.5 (s, 3H), 3.5–3.8 (m, 8H), 6.5–7.0 (m, 3H), 7.8–
7.9 (m, 4H), 8.3 (d, 4H), 8.9 (s, 1H), 9.3 (s, 1H). Anal. Calcd. for
3. Results and discussion
C27H24N8O7: C, 56.64; H, 4.19; N, 19.58. Found: C, 56.93%; H,
4.15%; N, 19.49%.
3.1. Synthesis and characterization of chromophores
2.4.6. Polyurethane IIc
The synthesis of new chromophores containing nitro-substi-
tuted thiazoles and oxadiazole was respectively achieved through
Wittig condensation and diazotization reactions. The chemical
FT-IR (KBr, cmꢀ1): 3308 (N–H), 3011, 2873 (Aliph.C–H), 1646
(C@O), 1606 (N@N), 1516, 1381 (N@O); 1H NMR (300 MHz,

20
R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
3300 cmꢀ1 corresponds to N–H stretching. The appearance of
broad N–H stretching suggests that the urethane linkages experi-
ence hydrogen bonding. The band appeared at around 1600 cmꢀ1
was attributed to C@N stretching, indicate the presence of isocya-
nate group in the polymer. The symmetrical and asymmetrical
stretching vibrations of nitro groups appeared respectively at
around 1516 and 1332 cmꢀ1 also clearly evident the presence of
NLO chromophores in the polyurethanes.
The 1H NMR spectrum of polyurethane IIa is presented in Fig. 3.
The spectrum of polyurethane showed signal broadening due to
polymerization. The signal resonated at around 8.5 ppm was as-
signed to amine protons, indicating the formation of urethane
linkages.
Inherent viscosity (g inh) of all the polymer solutions was mea-
sured at 25 °C and the resulting values were ranged between 0.26
and 0.30 dl/gl (see Table 1). Both analytical and spectral data are
consistent with the proposed polyurethane structures. All the poly-
urethanes were soluble in many of the common organic solvents
such as cyclohexanone, DMF, DMAc, DMSO and NMP, suggesting
that the resulting polyurethanes offer good processability, and
hence optical quality thin films could be easily prepared from their
solutions.
3.3. Thermal study
Thermal properties of the polyurethanes were examined by DSC
and TGA measurements. From the DSC traces shown in Fig. 4, it is
observed that glass transition temperatures were in the range of
121–192 °C. Among the polyurethanes, the polymers containing
chromophores b and c (Ib, IIb, Ic and IIc) exhibited relatively higher
Tg values compared to that of chromophore a (Ia and IIa). This is
mainly due to both bulkiness and polarity of the acceptor moieties.
The Tg values of all the polyurethanes are presented in Table 2.
Among the polyurethanes studied here, Ic exhibited the highest
glass transition temperature (Tg = 192 °C), reflecting the stable
NLO chromophore structure. On the other hand, polyurethanes
containing TDI backbone exhibited relatively higher Tg values com-
pared to those of polyurethanes containing MDI backbone. This can
be attributed to the enhanced chain rigidity caused by the incorpo-
ration of toluene ring.
Thermogravimetric analysis was carried out for all the polyure-
thanes and the patterns thus obtained are presented in Fig. 5. Irre-
spective of the chromophores, polyurethanes containing TDI
backbone (Ia, Ib and Ic) showed better thermal stability than those
of polyurethanes containing MDI backbone (IIa, IIb and IIc). This is
attributed to smaller chain length, which obviously increases the
rigidity of the polymers. This is in good agreement with Tg values
reported by the DSC. Thermal decomposition temperatures (Td)
of polyurethanes were also measured at the loss of 10, 20 and
40 wt.%, and the values thus obtained are included in Table 2. From
the data, it is observed that all the polyurethanes underwent
decomposition above 250 °C, which is far off from their corre-
sponding Tg values. This is the reason why, the poling temperature
fixed near the Tg values ( 10 °C) does not damage the thin films of
the NLO chromophores.
Fig. 10. Temporal stability of the SHG signals of polyurethanes at 100 °C in air.
structures of the chromophores were confirmed by FTIR, 1H NMR,
UV–vis and elemental analyses. The FTIR spectral data are consis-
tent with the proposed structures of chromophores. For instance,
the 1H NMR spectrum of chromophore a is presented in Fig. 1.
The spectrum of chromophore a showed two triplets. The signals
appeared at 3.6 and 3.8 ppm, which correspond to ethylene pro-
tons. The signals appeared at 6.6 and 7.1 ppm were assigned to stil-
bene protons. The signals resonated in the range of 7.3–8.4 ppm
were assigned to aromatic protons of the resulting chromophore.
All these ascertain the confirmation of reaction between N-phenyl-
diethanolamine and nitro-substituted aromatic ring.
The DMF solutions of chromophores a, b and c showed a strong
UV absorption maxima at around 409, 428 and 412 nm, respec-
tively. All the analytical data are in good agreements with the pro-
posed chemical structures. These chromophores were soluble in
many organic solvents such as acetone, cyclohexanone, DMF,
dimethylacetamide (DMAc), dimethylsulphoxide (DMSO) and N-
methyl-2-pyrrolidinone (NMP). These are the push–pulls type
NLO chromophores (see Scheme 1) with hydroxyl ethylamine as
the donor and nitro-substituted thiazole and oxadiazole groups
as the electron acceptors linked through stilbene and azo groups.
3.2. Synthesis and characterization of polyurethanes
3.4. Nonlinear optical properties of polyurethanes
The chromophores a, b and c were condensed with tolylene-2,4-
diisocyanate (I) and 4,40-methylenedi(phenyl isocyanate) (II) in a
dry DMF solvent to yield polyurethanes (see Scheme 2). The struc-
tures of the resulting polyurethanes were confirmed using FTIR, 1H
NMR, inherent viscosity and elemental analyses. The FTIR spectra
of only polyurethanes Ic, IIc, Ib, and IIb are presented in Fig. 2.
The characteristic band appeared at around 1700 cmꢀ1 was attrib-
uted to C@O stretching, indicate the establishment of urethane
The typical UV–vis absorption spectra of polyurethane IIa re-
corded before and after poling are presented in Fig. 6. After poling,
abrupt decrease in the absorbance was noticed. A similar behavior
was also noticed for other polyurethanes, and this happens gener-
ally because of two reasons: (i) a partial alignment of molecules
along the direction of the poling field, i.e., perpendicular to the
plane of the film, which reduces the absorption cross-section for
light at normal incidence [44], and (ii) a partial degradation of
bond in the polymer.
A strong band appeared at around

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
21
Fig. 11. The SHG signals of polyurethane IIb as a function of temperature.
the chromophores during the poling process. However, in the pres-
ent study the second reason is ruled out since the decomposition
temperatures of all polyurethanes (see Table 1) are well above
the poling temperature. Therefore, the abrupt decrease in the
absorbance is mainly due to alignment of NLO chromophores along
the direction of poling field. Further we notice that after the elec-
tric poling, the spectrum of polyurethane exhibited a blue shift,
and this is due to birefringence. Similar type of shift was also ob-
served in all the polyurethanes studied here.
polyurethanes were measured and data thus obtained are included
in Table 1. Fig. 8 illustrates the SHG signals of Ib as a function of
temperature. It is observed that the SHG signal was increased stea-
dily before Topt was reached, and then the SHG signals decreased.
From the Topt’s data, it is observed that optimal temperature of all
the NLO polyurethanes approaches their glass transition tempe-
ratures. Based on these Topt’s values, the noncentrosymmetric
alignment of the NLO chromophores in the polymer films was
induced with a corona poling method at a temperature close to
the glass transition temperature of the polymers.
Similar to Topt’s values, the set angle between polymer film and
path of laser beam is also playing a vital role in maximizing the
SHG intensity. Therefore, SHG measurements were performed
using a Nd:YAG laser at different angles. Fig. 9 shows the p-polar-
ized SH intensity of polymer film (Ia) versus the incident angle.
There is no Maker fringe structure, but only a big hump because
the film is so thin that the optical path length in the sample is
shorter than the coherence length. The SH intensity increases with
increasing the angle and reaches a maximum at h = 45° and there-
fore, for the routine measurements of second-order NLO coefficient
(d33) for all the samples, the incident angle was kept constant at
h = 45° and calculated the d33 values using the following equation
[47]:
From the change in absorbance, the order parameter of the
poled film could be estimated, which signifies the efficiency of
polling [45,46]. The order parameter (U = 1–A1/A0, where A0 and
A1 are the absorbances of the polymer films before and after poling,
respectively) of the poled polyurethane film (IIa) was found to be
0.34. The estimated order parameters of the remaining polyure-
thane films are included in Table 1. All these data suggest that
polyurethanes prepared in the present work were efficiently poled
and as a result, the chromophores were effectively aligned along
the direction of the applied field.
In order to justify the alignments of NLO chromophores along
the direction of field poling, we have investigated the domain
structure of NLO polymer films using atomic force microscopy.
Fig. 7 illustrate the AFM scans of the spin-coated film before and
after poling for polymer IIc. Before poling, the AFM image of IIc
shows that surface of the film is extremely flat and clean. However,
when it was subjected to poling, the good quality film was dramat-
ically changed, resulting to numerous hills and valleys in the sur-
face structure. This clearly justifies that after poling molecules
are effectively aligned along the direction of filed poling. Similarly,
we have taken AFM scans for all the remaining samples and all of
them demonstrated molecular alignments after the poling, but the
images are not presented to avoid the crowdness.
sffiffiffiffi
d33;s vðs2Þ
d36;k
Is l
Ik ls
¼
¼
c;k F
vðk2Þ
where d36,k is the d36 of KDP crystal, which is 0.40 pm/V; Is and Ik are
the SHG intensities of the polyurethane sample and the KDP crystal,
respectively; lc,k is the coherence length of the KDP crystal
(11.4 lm); ls is the thickness of the polyurethane sample; and F is
the correction factor, which is approximately equal to 1.2 when
lc,k ꢂ ls. The calculated d33 values of these polyurethanes at the
wavelength of 532 nm and the thickness of films were included in
Table 1. It is found that all the polyurethanes exhibited d33 values
in the range of 29.76–44.29 pm/V, indicating that polyurethanes
demonstrated an excellent performance in enhancing the second-
order NLO coefficients. All these d33 values are much higher than
those of dialkylaminonitrostilbene (DANS)-substituted NLO poly-
mer system [32,33,35,36]. From the results, we noticed that the
To achieve the maximum second-order NLO response in the
polyurethanes, selection of optimum poling temperature is crucial
so as to align the molecules to a maximum extent along the direc-
tion of the applied field. The in situ poling and temperature ramp-
ing technique were used to select the optimal temperature (Topt).
As the set poling temperature increased step by step, the SHG sig-
nal was detected at different temperature after the temperature
was held for 10–15 min. In this way, the Topt’s of the resulting

22
R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
polyurethanes having MDI backbone (IIa, IIb and IIc) demonstrated
better d33 values compared to TDI backbone regardless of the chro-
mophores attached. This suggests that MDI backbone favors better
molecular alignment during poling compared to TDI backbone. This
might be owing to the presence of flexible group (–CH2–) in the MDI
backbone. It is also noticed that among the polyurethanes, chro-
mophores containing oxadiazole moieties (Ic and IIc) exhibited lar-
ger d33 values compared to those of thiazole moieties (Ia, IIa, Ib and
IIb). This is clearly demonstrated that oxadiazole is a good acceptor
compared to thiazole moiety.
the second-order optical nonlinearity and thermal stability were
enhanced significantly. The d33 values ranged between 29.7 and
44.2 pm/V. High thermal endurance of the poled dipoles was ob-
served for all the polyurethanes and this was explained based on
the formation of extensive hydrogen bonds between urethane link-
ages. Because of the high solubility of polyurethanes, these could be
easily processed into high optical quality thin films for device
applications.
Acknowledgements
The relaxation process of dipole orientation is directly related to
the glass transition temperature (Tg). A higher Tg implies higher
orientation stability. To probe this stability, we monitored the tem-
poral and thermal stability of the SHG signals for all the polyure-
thanes at room temperature as well as at 100 °C. However, only
the data generated as a function of time at 100 °C are presented
in Fig. 10. All the polyurethanes were stable at room temperature
and absolutely there was no noticeable decay in the intensity of
SHG signals. However, at 100 °C in air, after an initial decay of less
than 5% of the original signal, more than 95% of the SHG signal re-
mained stable even up to measured time ꢁ400 h for all the poly-
urethanes regardless of the polymer backbones and
chromophores attached, suggesting that all the polyurethanes fa-
vor better stability at the performed temperature and time.
In order to determine the stability of polyurethanes at higher
temperature, we have studied the temporal stability of SHG signals
for all the polyurethanes. A typical example of polyurethane IIb is
only displayed in Fig. 11. To investigate the real time of NLO decay,
the SHG signal was monitored with a stepwise rise in temperature
from ambient to 240 °C. It can be clearly seen that at a temperature
near 160 °C the SHG signal started to decrease rapidly, suggesting
that thermal stability was observed up to 155 °C. This improved
stability is due to induced dipole alignment, which is mainly attrib-
uted to the hydrogen bondings. The formation of hydrogen bonds
in polyurethanes, polyamides, polypeptides, etc., is well known
and studied extensively [48–50]. These hydrogen bonds are known
to increase the rigidity of polymer systems while suppressing the
relaxation of aligned dipoles. Because our polyurethane systems
were completely amorphous, the high entropy conformation of
the backbone chain allowed the hydrogen bonding to take place
more freely. Thus, when our systems were compared with other
systems with similar structures and Tg values, such as poly(methyl
methacrylate) (PMMA) or polystyrene (PS)-based NLO polymers, in
which hydrogen bonding does not occur, better stability was ob-
served. A discussion of the stability of PMMA and PS-based NLO
polymers can be found in literatures [51,52]. From Fig. 11, it is fur-
ther noticed that a sharp decrease of SHG signal prevailed when
the temperature went beyond the Tg of polyurethane IIb. However,
it was found that the same magnitude of the SHG signal could be
recovered from the same sample after it was re-poled by corona
discharging, again suggesting that the NLO chromophore was not
damaged even after experiencing high temperatures. Similar re-
sults were observed in the remaining polyurethanes Ia, IIa, Ib, Ic
and IIc. These thermal characteristics and their optical nonlineari-
ties clearly indicate that these polyurethanes are useful in device
applications.
The authors gratefully acknowledge the Department of Physics,
Indian Institute of Science, Bangalore, and the Chemistry & Physics
of Materials Units, Jawaharlal Nehru Center for Advanced Scientific
Research, Bangalore, for providing the Ellipsometer and Atomic
Force Microscopy (AFM) to measure the refractive indices, thick-
ness and surface morphology of thin films. One of the authors
(RGT) wishes to acknowledge the UGC, New Delhi for awarding
the Research Fellowship under meritorious category.
References
[1] H.N. Jang, H.J. No, J.Y. Lee, B.K. Rhee, K.H. Cho, H.D. Choi, Dyes Pigments 82
(2009) 215.
[2] M.J. Cho, S.K. Lee, J.I. Jin, D.H. Choi, Macromol. Res. 14 (2006) 609.
[3] K.R. Yoon, S.O. Ko, S.M. Lee, H. Lee, Dyes Pigments 75 (2007) 573.
[4] X. Chen, J. Zhang, H. Zhang, Z. Jiang, G. Shi, Y. Li, Dyes Pigments 77 (2008) 228.
[5] M.J. Cho, S.K. Lee, D.H. Choi, J.I. Jin, Dyes Pigments 77 (2008) 342.
[6] J.A.F. Boogers, P.T.A. de Klaase, J.J. Vligere, A.H.A. Tinnemans, Dyes Pigments 27
(1994) 209.
[7] M. Trollsas, C. Orrenius, F. Sahlen, U.W. Gedde, T. Norin, A. Hult, J. Am. Chem.
Soc. 118 (1996) 8548.
[8] K.S. Hans, S.K. Park, S.Y. Shim, W.S. Jang, N.J. Kim, Bull. Korean Chem. Soc. 19
(1998) 1168.
[9] K.S. Hans, S.K. Park, S.Y. Shim, W.S. Jang, N.J. Kim, Bull. Korean Chem. Soc. 19
(1998) 1171.
[10] J. Xie, X. Deng, Z. Cao, Q. Shen, W. Zhang, W. Shi, Polymer 48 (2007) 5993.
[11] N. Marioara, J. React. Funct. Polym. 69 (2009) 35.
[12] J. Hao, M.J. Han, K. Guo, Y. Zhao, L. Qiu, Y. Shen, Mater. Lett. 62 (2008) 976.
[13] A. Carella, A. Castaldo, R. Centore, A. Fort, A. Sirigu, J. Chem. Sci. Perkin Trans. 2
(2002) 1795.
[14] R.M.F. Batista, S.P.G. Costa, M. Belsley, M.M. Raposo, Tetrahedron 63 (2007)
9849.
[15] V.P. Rao, A.K.–Y. Jen, K.Y. Wong, K.J. Drost, Tetrahedron Lett. 34 (1993) 1750.
[16] A. Abbotto, S. Bradamante, A. Facchetti, G.A. Pagani, Facile, J. Org. Chem. 62
(1997) 5765.
[17] S. Yuquan, Z. Yuxia, L. Zao, W. Jianghong, Q. Ling, L. Shixiong, J. Chem. Soc.
Perkin Trans. 1 (1999) 695.
[18] C.W. Dirk, H.E. Katz, M.L. Schilling, L.A. King, Chem. Mater. 2 (1990) 705.
[19] S.M. Tambe, A.A. Kittur, S.R. Inamdar, G.R. Mitchell, M.Y. Kariduraganavar, J.
Opt. Mater. 31 (2009) 825.
[20] W.N. Leng, Y.M. Zhou, Q.H. Xu, J.Z. Liu, Polymer 42 (2001) 9259.
[21] P. Ambrosanio, R. Centore, S. Concilio, B. Panunzi, A. Sirigu, N. Tirelli, Polymer
40 (1999) 4928.
[22] P. Zhu, E. Milko, Vander Boom, Kang Hu, G. Evmenenko, P.G. Dutta, Chem.
Mater. 14 (2002) 4989.
[23] Z. Li, J. Hua, Q. Li, C. Huang, A. Qin, C. Ye, Qin, Polymer 46 (2005) 11948.
[24] M. Dudkina, V. Aseev, A. Tenkovtsev, H. Tenhu, J. Polym. Sci. B Polym. Phys. 43
(2005) 404.
[25] A. Qin, Z. Yang, F. Bai, C. Ye, J. Polym. Sci. A Polym. Chem. 41 (2003) 2853.
[26] J. Lu, J. Yin, J. Polym. Sci. A Polym. Chem. 41 (2003) 312.
[27] S.D. Reddy, C.F. Shu, F.I. Wu, J. Polym. Sci. A Polym. Chem. 40 (2002) 268.
[28] S.H. Hsiao, Y.J. Chen, Eur. Polym. J. 38 (2002) 828.
[29] J. Xu, C. He, T.S. Chung, J. Polym. Sci. A Polym. Chem. 39 (2001) 3007.
[30] H. Saadeh, L. Wang, L. Yu, Macromolecules 33 (2000) 1576.
[31] Y. Sakai, M. Ueda, T. Fukuda, H. Matsuda, J. Polym. Sci. A Polym. Chem. 37
(1999) 13219.
[32] W. Ballet, T. Verbiest, M.V. Beylen, A. Persoons, C. Samyn, Eur. Polym. J. 37
(2001) 2424.
4. Conclusions
[33] J.Y. Lee, H.B. Bang, E.J. Park, C.S. Baek, B.K. Rhee, S.M. Lee, Synth. Metals 144
(2004) 167.
[34] U. Caruso, M. Casalboni, A. Fort, M. Fusco, B. Panunzi, A. Quatela, J. Opt. Mater.
27 (2005) 1810.
[35] J.Y. Lee, H.B. Bang, C.S. Baek, Synth. Metals 148 (2005) 168.
[36] J.Y. Lee, H.B. Bang, H.B. Bang, B.K. Rhee, C. Jung, S.M. Lee, Bull. Korean Chem.
Soc. 24 (2003) 1727.
[37] H.Y. Woo, H.G. Shim, K.S. Lee, Polymer 32 (2000) 14.
[38] J.Y. Lee, E.J. Park, H. Lee, B.K. Rhee, Polym. Bull. 48 (2002) 241.
NLO responsive polyurethanes containing nitro-substituted
oxadiazole and thiazole moiety in the side chain of the chromoph-
ores were synthesized and characterized. The polyurethanes having
TDI backbone demonstrated relatively higher Tg values compared to
MDI backbone. This is due to the enhancement of rigidity caused by
the incorporation of toluene ring. After orientation of the dipoles,

R.G. Tasaganva et al. / Journal of Molecular Structure 1000 (2011) 10–23
23
[39] C.K. Park, C.F. Zieba, C.F. Zhao, B. Swedek, W. Wijekoon, Macromolecules 28
(1995) 3717.
[46] A. Hayashi, Y. Goto, M. Nakayama, K. Kaluzynski, H. Sato, K. Kato, K. Kondo, T.
Watanabe, S. Miyata, Chem. Mater. 4 (1992) 52.
[40] N. Tsutsumi, S. Yoshizaki, W. Sakai, T. Kiyotsukuri, Macromolecules 28 (1995)
6442.
[47] L.R. Dalton, C. Xu, A.W. Harper, R. Ghosn, B. Wu, Z. Liang, Sect. B: Nonlinear
Opt. 10 (1995) 389.
[41] S.-Y. Song, M.S. Jang, Macromolecules 32 (1999) (1482).
[42] R.G. Tasaganva, M.Y. Kariduraganvar, S.R. Inamdar, Synth. Metals 159 (2009)
1819.
[48] R. Bonart, J. Macromol. Sci. Phys. B 2 (1968) 138.
[49] R.B. Seymour, Polymer Chemistry, second ed., Marcel Dekker, New York, 1998.
p. 175.
[43] S.M. Tambe, R.G. Tasaganva, J.J. Jogul, D.C. Castiglione, M.Y. Kariduraganavar, J.
Appl. Polym. Sci. 114 (2009) 2300.
[44] R. Pizzoferrato, F. Sarcinelli, M. Angeloni, M. Casalboni, F. Bertinelli, P.B. Costa,
Chem. Phys. Lett. 343 (2001) 211.
[45] M.A. Mortazavi, A. Knoesen, S.T. Kowel, B.G. Higgins, A. Dienes, J. Opt. Soc. Am.
B 6 (1989) 733.
[50] C. Ye, T.J. Marks, J. Yang, G.K. Wong, Macromolecules 20 (1987) 2324.
[51] D.R. Robello, P.T. Dao, J.S. Schildkraut, M. Scozzafava, E.J. Urankar, C.S. Willand,
Chem. Mater. 7 (1995) 291.
[52] B. Joglekar, K. Shibata, H. Muramatsu, M. Matsui, K. Hirota, M. Hosoda, Polymer
29 (1997) 187.